Crack initiation and viscoplasticity in polyethylene joint replacement components

Ultrahigh molecular weight polyethylene (UHMWPE) is an abrasion resistant and bioinert polymer widely used as a bearing material in total joint replacements. Recent reports of fracture and crack initiation in these systems make the prediction of crack initiation a primary concern. Past work in assessing the resistance to crack propagation in UHMWPE has typically ignored the creeping (quasi-static) constitutive contribution to the process of failure. We conducted constant load experiments on pre-notched fracture specimens and observed the elapsed time to crack initiation and subsequent crack velocity as a function of the applied load. A hyperelastic-viscoplastic constitutive model was calibrated to three uniaxial tensile experiments; one at a constant crosshead velocity, and two delayed yield creep (viscoplastic) experiments at an engineering stress either slightly or moderately below the short-term yield strength, until the strain saturated or failure. The crack initiation phase of the fracture experiment was modeled in ABAQUS, predicting the time-dependent J-integral and average molecular chain stretch to be single-valued at the experimentally obtained crack initiation times for the three tested boundary conditions. The crack initiation time and propagation velocity were also found to scale with the applied load in agreement with an analytical power-law viscous fracture model.